Refractory materials generally consist of non-metallic ceramic substances characterized by their suitability for use as structural materials at high temperatures, usually in contact with metals, slags, glass or other corrosive materials. Refractories are classified chemically as acid, basic, or neutral, are found typically in a raw-mined, process-fired, or chemically bonded state, and may be used in mass, granular or finely divided form. Refractory materials are further variously classified according to the raw materials employed and the minerals contained therein. As such, refractories have been identified by groups as siliceous, fire clay, high-alumina, magnesium-silica, magnesia-lime, chromite, and carbon.
Chrome ore, or chromitite, containing chromite and other gange materials is typically used as a refractory in a raw state in granular form as ground in open-hearth front wall maintenance, or for reheating-furnace hearths and open-hearth doors in plasticized form. Certain chrome ore refractories are described and disclosed in the following U.S. Pat. Nos.: 308.932, 1,437,584, 1,911,189, 2,792,311, 2,809,126.
Refractories are widely employed in iron and steel making operations as insulation in furnaces, flues, stacks, as runners, and as linings of ladles, pots, crucibles, and other high temperature material-receiving vessels. A detailed discussion of such refractory materials, their compositions, classifications, and uses is found in a 1971 U.S. Steel book entitled The Making, Shaping and Treating of Steel.
In addition to iron and steelmaking, refractories are also employed in other related high temperature processing operations, such as in the electric smelting process for production of chromium alloys of high and low carbon ferrochromium. Ferrochromium is the principal alloy used in the production of cast irons, stainless steel and other specialty steels. Chromitite, the only commercial ore of chromium, is a material in which iron oxide and chromium oxide exist in a combined form. The mineral chromite (FeCr.sub.2 O.sub.4) has a spinel structure and exists with other gangue material in the chromitite ore. Chromite is most economically reduced by using carbon in an electric furnace to produce what is known as high-carbon ferrochromium. High-carbon ferrochromium, or charge chrome as it is also known, is made to many specifications, varying in chromium content from about 50% to 75%, in carbon content of from about 4% to 10%, and in silicon content of less than 1% to as much as about 10%. In distinction, low-carbon ferrochromium alloys generally possess a maximum carbon content of less than about 2% and are usually made by duplex or triplex processes involving de-siliconization of ferrochrome-silicon with a fluid chrome ore-lime slag.
The American ASTM specification A-101-50 characterizes high and low carbon ferrochromium by content, as follows:
______________________________________ High-carbon (%) Low-carbon (%) ______________________________________ Cr 60 to 75 65 to 75 C 4 to 6 2 (max) Si 3 1.5 ______________________________________
High carbon ferrochromium manufactured by direct reduction of chromite with carbon in an electric furnace is known as submerged-arc smelting. Electrodes (usually three) are submerged in a burden consisting of one or more chromium ores, coke, and fluxes, such as quartz, kaolin, limestone, and the like. The mix is fed at the top of the furnace and around the three electrodes. In the smelting operation, the iron and chromium oxides are reduced to produce molten metal while the other gangue constituents of the ore which are lighter than the molten metal form a molten slag by-product. For proper fluidity, the slag should contain about 26 to 32% silica (SiO.sub.2), and silica-containing materials generally are added to the ore mixture to provide the desired consistency. In smelting operations involving formation of slag, the burden supplied to the electric furnace is prepared using a calculated metallurgical balance to ensure that the chromitite will give the desired alloy composition and a workable slag by-product for separation.
A submerged-arc electric furnace is utilized in the manufacture of high carbon ferrochromium because of the high temperature required for the chemical reaction of carbon with chromite. The arc between the electrode, which is made up of baked coal, coke, and pitch, and the charge is buried or submerged in the burden or mix and much of the heat is generated by the resistance of the burden. Modern production furnaces are mostly three-phase, employing three electrodes in triangular formation. The molten metal and slag gather low in the furnace, perhaps four to six feet below the arc. The arc temperature is approximately 3680.degree. C.; however, the temperature of the molten metal and slag would be lower depending on the carbon content of the high-carbon ferrochromium, as indicated in the following table:
______________________________________ STABILITY RANGES FOR PRODUCTS OF REDUCTION OF Cr.sub.2 O.sub.3 PRODUCT Ferrochromium RANGE .degree.C. % CARBON IN ______________________________________ Cr.sub.7 C.sub.3 1250 to 1600 9.0 Approximately Cr.sub.23 C.sub.6 1600 to 1820 5.5 to 6.0 ______________________________________
For manufacture of ferrochromium alloys with 5.5 to 6.5% carbon content, the temperature of the metal and slag as they are tapped from the furnace ranges from 1600.degree. to 1820.degree. C.
The constituents of chrome ore are essentially Cr.sub.2 O.sub.3, FeO, Al.sub.2 O.sub.3, MgO, CaO and SiO.sub.2. The valuable minerals in the chromium ore are Cr.sub.2 O.sub.3 and FeO, which are reduced to chromium and iron in the electric furnace, and make up the high-carbon ferrochromium alloy sought. The Al.sub.2 O.sub.3, MgO, CaO and SiO.sub.2 in the chromium ore are gangue materials and form a portion of the slag in the electric furnace.
The composition of the coke is essentially carbon and ash. The carbon in the coke combines with the oxygen in the Cr.sub.2 O.sub.3 and FeO, thereby freeing the FeCr. The chemical reactions are as follows:
Cr.sub.2 O.sub.3 +3C 2Cr+3CO (Gas) PA1 FeO+C Fe+CO (Gas)
The ash in the coke, usually 10 to 12%, and made up mainly of Al.sub.2 O.sub.3, MgO and SiO.sub.2, also forms a portion of the slag.
Fluxes, such as quartz (SiO.sub.2), kaolin (Al.sub.2 O.sub.3 and SiO.sub.2), and limestone (CaCO.sub.3), generally are added to condition the slag and give the metallurgical balance desired.
Thus, the gangue from the chromitite, the gangue from the coke (the ash), and the additional fluxes make up the molten slag composition in the electric furnace.
In the chromitite smelting operation, the smelting furnace is periodically tapped at the bottom of the burden to discharge a molten metal and slag mixture into a refractory brick-lined receiving vessel or ladle. The heavier molten metal settles to the lower portion of the receiving ladle and the lighter molten slag rises to the top, where it flows over the top of the first receiving ladle into a second vessel, generally referred to as a slag pot. This separates the slag from the molten metal alloy product. Conventionally, the molten slag by-product in the slag pot is poured into a slag pit where it solidifies and is broken up into small pieces. These slag pieces are hydraulically concentrated in a jig for further recovery of small metal alloy bits and pieces which may have been carried over into the slag during the metal/slag separation step. The final slag by-product of the high carbon ferrochromium smelting process has occasionally heretofore been further fragmentized and sold for use as a roadway aggregate, foundation fill, berme, or driveway surface.
Because of the high temperatures employed in high-carbon ferrochromium smelting, as well as steel-making and other molten ore and metal processing operations, it is a conventional practice to employ refractory materials as interior walls and insulating linings of furnaces, receiving vessels, runners, ladles, and other surfaces which contact and contain high temperature materials. Such refractory materials have generally been of the types hereinabove described, and are structurally installed in the form of bricks, blocks, or other cast shapes to line the inside of metal outer support components of furnaces, receiving vessels, ladles, and the like to protect against damage and burn-out from the high temperature materials being handled.
The installation and use of refractory materials as inner walls and linings of high temperature processing equipment adds to the initial cost of such equipment, and also creates an operating cost for inventory requirements and periodic replacement of the refractory due to degradation and wear during manufacturing operations.